1. Field Of The Invention
This invention relates to graft copolymers of polyvinylidene fluoride and to methods for their preparation. More especially this invention relates to graft copolymers of polyvinylidene fluoride and ethylenically unsaturated compounds especially vinyl compounds and olefins. This invention is also directed to a process for the preparation of such copolymers by contacting polyvinylidene fluoride in the solid phase with a radical forming catalyst and an ethylenically unsaturated monomer maintaining the ethylenically unsaturated compound at polymerization conditions while in the presence of said radical forming catalyst.
2. Discussion Of The Prior Art
It is known that polyvinylidenefluoride (PVDF) has a very good resistance to chemicals and therefore is used in the manufacture of products which are exposed to aggressive chemicals. On the other hand, unmodified PVDF also has a number of undesirable characteristics which in many cases conflict with its application, or make it difficult or impossible to fabricate. For example, the ability of unmodified PVDF to adhere to plastics or even to many metals is poor, so that adhesivizing agents must be used, which result in a certain improvement of adhesion. When adhesivizing agents are used, however, and especially when thin films are to be applied to a substrate, it is often found that the adhesivizing agent migrates into the PVDF and adversely affects its chemical resistance.
The poor adhesion proves especially troublesome in the production of laminates or extruded products. In the presence of heat there is a bond, but upon cooling the bond is lost, so that without the addition of adhesivizing agents such bonding is virtually impossible.
The modification of PVDF with other fluoropolymers does not solve the problem. Furthermore, it is known, for example, through U.S. Pat. No. 3,790,645, to prepare polymers from PVDF and methyl methacrylate, whose electrical characteristics are better than those of unmodified PVDF. There is no mention in this disclosure, however, of any improvement of the adhesiveness of these polymers. These polymers are prepared in aqueous phase, the polymerization being performed in the presence of wetting agents and of the same catalyst which is used in the polymerization of the vinylidene fluoride. The polymer that is obtained by this process, however, has the disadvantage that it is attacked by the solvents for the second polymer. For example, in the case of a PVDF copolymerized with polymethylmethacrylate, virtually all of the polymethylmethacrylate is dissolved out of it by tetrahydrofuran, so that in a polymer of this kind some of the physical properties of the on-polymerized material persist.
It is an object of this invention, therefore, to provide a PVDF polymer having improved adhesivizing characteristics. It is another object of this invention, therefore, to provide a PVDF polymer composition which will more readily adhere to substrates such as inorganic oxidic or metallic surfaces with or without assistance from an adhesivizing agent. It is an object of this invention furthermore, to provide a simple means for the preparation of such PVDF composition.